I was just in the process of replacing the RO membranes on my Village Marine (aka Offshore Marine) watermaker and found out that the membranes that they use aren't standard. Their membrane is labeled "SW-2540" which stands for Salt Water 2.5"x40", or that's what I thought. It's the standard nomenclature used for membranes, but guess what, not for them! Their membranes are 38" and come with a 2" extender for use in standard pressure vessels. Unfortunately I have one of their pressure vessels and it only uses 38" membranes. And guess what? Their membranes cost $570 vs the normal $185 price for a 40" membrane!
Well, I said FU and am replacing their pressure vessels with standard pressure vessels. It'll cost me more this time, but next time I'll be ahead.
For just this reason I'd never buy a Village Marine system again. Talk about locking you into expensive proprietary parts! However, I will say that their systems that use 2521 (21") membranes use industry standard parts.
-- Geoff